The Free On-line Dictionary of Computing (19 January 2023):

pseudoprime

    A number that is probably a prime number, as
   determined using statistical methods.

   A backgammon "prime" is six consecutive points that are "made"
   (each has two or more pieces of same colour).  A pseudoprime
   might then describe six consecutive points, five of which are
   made which, like the mathematical concept, is often good
   enough.

   [Jargon File]

   (2020-11-21)


The Jargon File (version 4.4.7, 29 Dec 2003):

pseudoprime
 n.

    A backgammon prime (six consecutive occupied points) with one point
    missing. This term is an esoteric pun derived from number theory: a number
    that passes a certain kind of “primality test” may be called a pseudoprime
    (all primes pass any such test, but so do some composite numbers), and any
    number that passes several is, in some sense, almost certainly prime. The
    hacker backgammon usage stems from the idea that a pseudoprime is almost as
    good as a prime: it will do the same job unless you are unlucky.